In various video codecs, motion estimation/motion compensation is a key technology that affects encoding performance. In existing various video codecs, assuming that an object always holds a translation motion, and that all parts of the entire object move in a same direction and at a same speed, a block-based motion compensation technique is used. However, all these methods are based on a translational motion model, and are improvements made based on a block-based motion compensation method. In the real world, motions are diversified. Irregular motions such as an affine motion, scaling, rotation, and shearing are universal. Existence of these motions spoils motion prediction effects of block-based motion compensation. Since the 1990s, video encoding experts realized that irregular motions are universal, and introduced an irregular-motion model, such as an affine motion model, to improve video encoding efficiency.
In the conventional art, an affine motion model is used for encoding. According to a rate-distortion optimization principle, Lagrangian costs in using the affine parameter model for encoding a current block may be compared with costs in using another model in an encoder. If the costs in encoding by using the affine parameter model are less than the costs in another encoding model, the affine parameter model is finally selected for encoding the current block. A quantized affine model parameter is encoded into a bitstream, so that a decoder can use the parameter to reconstruct the current block. However, in the conventional art, a six-parameter affine motion parameter model needs to be used, that is, additional bits in each block are required to encode six affine motion model parameters. This restricts further improvements of encoding efficiency.